The present invention relates to gas turbine engines, particularly aero gas turbine engines.
One problem with gas turbine engines is the thermal distortion which occurs when a gas turbine engine is shut down after use. The residual heat in the various components of the gas turbine engine causes convection currents to be set up which cause the upper portions of the gas turbine engine to retain their heat for longer than the lower portions of the gas turbine engine. This produces a temperature differential which in turn caused differential thermal expansion.
The effect of the differential thermal expansion is to cause at least the rotors, particularly the high pressure rotor, to bow upwardly. The amount of rotor bow is time dependent. For a given heat content within the gas turbine engine, the maximum rotor bow will occur some time after shut down, when the convective heat transfer has had time to act, but before the gas turbine engine has cooled down. The magnitude of the temperature differential between the upper portion and the lower portion of the gas turbine engine and the magnitude of the rotor bow depends on the heat content of the gas turbine engine, so that when the gas turbine engine has cooled down the temperature differential and rotor bow disappear.
The distortion, or bowing, of the rotor in itself is not harmful to the gas turbine engine. However, if it is desired to restart the gas turbine engine while the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion the displacement of the centre of mass of the distorted rotor from the centre of rotation may create problems.
The first problem is large, damaging vibrations of the rotor and possibly rubbing of the rotor with the surrounding stator when the rotor passes through its first critical speed because the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion. It is normal practice to arrange for the first critical speed of the rotor to be less than the idle speed. The rotor typically comprises two portions which are connected by a spigotted, bolted, joint. As the rotor cools down the spigotted, bolted, joint may become loose due to the differential thermal expansion and hence the vibrations of the rotor may produce wear at the spigotted, bolted, joints. The worn spigotted, bolted, joints exacerbate the vibrational response of the gas turbine engine rotor to rotor bowing.
The second problem is additional stresses are produced in the rotor when the rotor reaches high speed operation after start up if the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion. The gas turbine engine may be started from a cooling condition and accelerated to idle speed and then to high speed before the rotor has warmed through to a uniform temperature circumferentially around the rotor. This is because of the high thermal inertia of the rotor discs and drums. The effect of the rotor bow is to superimpose an extra stress onto the high stress levels in the rotor, thus some circumferential parts of the rotor will have an additional tensile stress and some circumferential parts will have an additional compressive stress. The result is that the expected service life of a rotor that is frequently started in a bowed condition is less than that of a rotor that is never started in a bowed condition.
UK patent application GB2117842A discloses the use of ducts and blowers to circulate warmer gas from the upper portion of the gas turbine engine to the lower portion of the gas turbine engine or circulate cooler gas from the lower portion of the gas turbine engine to the upper portion of the gas turbine engine. This requires the provision of ducts and blowers to the gas turbine engine which adds weight and cost to the gas turbine engine.
UK patent application GB2117450A discloses the use of a particular mounting arrangement for the compressor casing and heaters to differentially heat the mounting to displace the casing to compensate for the distortion of the rotor. This requires the provision of the particular mounting and heaters which adds weight and cost to the gas turbine engine and does not solve the problem of vibration of, the rotor.
Accordingly the present invention seeks to provide a novel component for a gas turbine engine which overcomes the above mentioned problems.
Accordingly the present invention provides a gas turbine engine comprising at least one rotor and at least one casing arranged coaxially around the at least one rotor, at least one of the at least one rotor or the at least one casing having a low emissivity surface finish, or high emissivity surface finish, on at least a portion of its surface to reduce temperature differentials between an upper portion and a lower portion of the at least one rotor.
Preferably the gas turbine engine comprises a low pressure rotor and a high pressure rotor arranged coaxially around the low pressure rotor and at least one casing arranged coaxially around the high pressure rotor, at least one of the high pressure rotor, the low pressure rotor or the at least one casing having a low emissivity surface finish, or high emissivity surface finish, on at least a portion of its surface to reduce temperature differentials between an upper portion and a lower portion of at least one of the low pressure rotor and the high pressure rotor.
The high emissivity surface finish may be arranged on substantially the whole of the radially inner surface of the high pressure rotor. The high emissivity surface finish may be arranged on substantially the whole of the radially inner surface of the low pressure rotor. The high emissivity surface finish may be arranged on substantially the whole of the radially outer surface of the low pressure rotor.
A low emissivity surface finish may be arranged on at least a portion of the radially outer surface of the high pressure rotor and a low emissivity surface finish is arranged on a portion of the radially inner surface of the high pressure rotor.
A high emissivity surface finish may be arranged on at least a portion of the radially inner surface of the high pressure rotor, a high emissivity surface finish is arranged on at least a portion of the radially outer surface of the high pressure rotor, a low emissivity surface finish is arranged on at least a portion of the radially outer surface of the high pressure rotor and a low emissivity surface finish is arranged on at least a portion of the radially inner surface of the high pressure rotor.
The high emissivity surface finish may be arranged on at least a portion of the downstream surface of the high pressure rotor. The high emissivity surface finish may be arranged on substantially all of the downstream surface of the high pressure rotor.
The high emissivity surface finish may be arranged on at least a portion of the downstream surface of the low pressure rotor. The high emissivity surface finish may be arranged on substantially all of the downstream surface of the low pressure rotor.
The high pressure rotor may comprise a high pressure compressor and a high pressure turbine and the low pressure rotor comprises a low pressure compressor and a low pressure turbine.
The high pressure turbine may comprise at least one turbine disc and the low pressure turbine comprises at least one turbine disc.
The high emissivity surface finish may be arranged on the upstream surface of each turbine disc on the high pressure rotor and the downstream surface of each turbine disc on the high pressure rotor.
The high emissivity surface finish may be arranged on the upstream surface of each turbine disc on the low pressure rotor and the downstream surface of each turbine disc on the low pressure rotor.
The at least one casing may be arranged around the high pressure turbine and the low pressure turbine, at least a portion of the at least one casing having a high emissivity surface finish on its outer surface to increase the rate of radiation of heat from at least one of the turbines.
At least a portion of the at least one casing may have a high emissivity surface finish on its outer surface. At least a portion of the at least one casing may have a high emissivity surface finish or a low emissivity surface finish on its inner surface.
The high emissivity surface finish may comprise a coating applied to, or formed on, the surface of the at least one rotor or the at least one casing. The coating may comprise a high emissivity metal oxide, a metal oxide formed on the at least one rotor or the at least one casing due to oxidation of the at least one rotor or the at least one casing, carbon, black paint or other suitable colour paint.
The low emissivity surface finish may comprise a polished or machined portion of the surface of the at least one rotor or the at least one casing, or a coating applied to the surface of the at least one rotor or the at least one casing. The coating may comprise a polished metal coating, a polished silver coating, a polished gold coating, or a low emissivity metal oxide.
The at least one rotor may be rotatably mounted on the casing by a support structure, the support structure carrying a bearing chamber having a bearing.
The surface of the bearing chamber having a low emissivity surface finish. The upstream and downstream surfaces of the support structure having a high emissivity surface finish.
The present invention also provides a rotor for a gas turbine engine having a low emissivity surface finish, or high emissivity surface finish, on at least a portion of its surface to reduce temperature differentials between an upper portion and a lower portion of the rotor.
A high emissivity surface finish may be arranged on at least a portion of the radially inner surface of the rotor. The high emissivity surface finish may be arranged on substantially the whole of the radially inner surface of the rotor. A high emissivity surface finish may be arranged on at least a portion of the radially outer surface of the rotor. The high emissivity surface finish may be arranged on substantially the whole of the radially outer surface of the rotor.
A low emissivity surface may be arranged on at least a portion of the radially outer surface of the rotor. The low emissivity surface finish may be arranged on substantially the whole of the radially outer surface of the rotor.
A high emissivity surface finish may be arranged on at least a portion of the radially inner surface of the rotor, a high emissivity surface finish is arranged on at least a portion of the radially outer surface of the rotor, a low emissivity surface finish is arranged on at least a portion of the radially outer surface of the rotor and a low emissivity surface finish is arranged on at least a portion of the radially inner surface of the rotor.
A high emissivity surface finish may be arranged on at least a portion of the downstream surface of the rotor. The high emissivity surface finish may be arranged on substantially all of the downstream surface of the rotor.
The rotor may be a high pressure rotor, an intermediate pressure rotor or a low pressure rotor.
The high emissivity surface finish may comprise a coating applied to, or formed on, the surface of the rotor. The coating may comprise a high emissivity metal oxide, a metal oxide formed on the rotor due to oxidation of the rotor, carbon, black paint or other suitable colour paint.
The low emissivity surface finish may comprise a polished or other machined portion of the surface of the rotor or a coating applied to the surface of the rotor. The coating may comprise a polished metal coating, a polished silver coating, a polished gold coating, or a low emissivity metal oxide.